Recently, surface mount electronic components such as IC, transistor, diode, condenser, piezoelectric element resistor and the like have been packed into a package composed of a plastic carrier tape having successively formed therein emboss-molded pockets into which the electronic components can be received in conformity with the shapes of them, and a cover tape heat-sealable to the carrier tape, and then supplied. The electronic components packed are automatically taken out from the package after the cover tape has been peeled from the carrier tape, and then surface mounted on an electronic circuit substrate.
The strength for peeling the cover tape from the carrier tape is called "peel-off strength", and when this strength is too low, there is such a problem that the cover tape gets off during the transportation of the package and the electronic components packed fall off. On the other hand, when the strength is too high, there has been caused such a phenomenon that the carrier tape vibrates when the cover tape is peeled and the electronic components jump out of the storing pockets just before they are mounted, that is, a jumping trouble has been caused.
The mechanism by which the cover tape now on the market peels from the carrier tape is classified into three types of interfacial peeling type, transfer-peeling type and cohesive failure type. The interfacial peeling type refers to such a type that the cover tape and the carrier tape are peeled from each other at the sealing face, the transfer peeling type refers to such a type that the bonding layer per se is transferred to the carrier tape during the peeling, and the cohesive failure type refers to such a type that a layer different from the bonding layer or the bonding layer per se (the two being referred to hereinafter as the cohesive failure layer) is broken to cause peeling. Each of these types has both merits and demerits; however, when they are compared only in respect of the state in which the cover tape sealed to the carrier tape is peeled, the interfacial peeling type tends to be affected by the shape, material and properties of the carrier tape because the sealing face and the peeling face are the same, and hence the peel-off strength tends to become instable.
In the case of the transfer-peeling type, the bonding layer must be a thin film in view of the mechanism and the so-called heat-sealing lacquer must be used. Therefore, the peel-off strength tends to become sensitive to sealing temperature, and hence sealing conditions for a suitable peel-off strength are hardly obtained.
In the case of the cohesive failure type, the sealing face and the peeling layer are different, and hence the dependency of the peel-off strength upon the sealing conditions is low. Also, the cohesive failure type has such a great advantage that the peel-off strength is not affected by the shape, material and properties of the carrier tape. However, in some cases, during the peeling, the cohesive failure layer is affected by a layer other than the cohesive failure layer including the bonding layer and interfacial peeling takes place without causing cohesive failure. Also, it is difficult to decide the position at which the cohesive failure layer is broken, and hence the cohesive failure layer remains on the surface of the carrier tape during the peeling and there is brought about such a state that the contents cannot be taken out.
The cohesive failure layer per se is designed so as to be easily broken, and hence in many cases, it is composed of a mixture of a plurality of resins which are hardly miscible with one another and these resins are not mixed uniformly. This results in a deterioration of the transparency of the cover tape and in a disadvantage due to agglomerates in some cases. Also, in such uses, a resin inferior in heat resistance is contained in the resin mixture in some cases. For these reasons, the agglomerates or degradation products appear during the formation of the cohesive failure layer and decrease the productivity in many cases. For example, Japanese Patent No. 1,347,759 (Applicant: UCB Societe Anonyme) recites in claim 5 a blend of polyethylene, polystyrene and an elastomeric styrene-butadiene-styrene or styrene-isoprene-styrene block copolymer, and when the blend is used to try the formation of the cohesive failure layer, the butadiene or isoprene component causes polymerization reaction when the processing temperature exceeds 200.degree. C. to produce an agglomerate, resulting in a remarkable decrease of the production yield.